In the United States, one in every three individuals is considered very overweight or obese. More than a cosmetic issue, obesity is a positive correlate to cardiovascular disease, hypertension, atherosclerosis and diabetes. The discovery of leptin as an adipocyte-derived secreted cytokine has expanded our view of the fat cell beyond being a repository for caloric excess to a dynamic cell type functioning in the assessment and integration of overall energy homeostasis. The focus of the laboratory is on the linkage between obesity, fatty acid metabolism and lipolysis with an emphasis on fatty acid trafficking in the adipocyte and NIDDM. The laboratory studies fatty acid carriers of the adipocyte, specifically the products of the FABP4 and FABP5 genes. In fat cells, these proteins form 1:1 non-covalent complexes with fatty acids and other hydrophobic monoacyl lipids. aP2 (FABP4) and KLBP (FABP5) each form a physical complex with the hormone-sensitive lipase (HSL), the key rate-limiting enzyme of lipolysis that hydrolyzes triacylglycerol to fatty acids. Consistent with the association of aP2 with the HSL, FABP4 null mice exhibit reduced basal and hormone-stimulated lipolysis suggesting fatty acid binding proteins affect the process by removing end-product fatty acids from the lipase reaction and facilitating their efflux from the cell. FABP4 null mice exhibit protection from obesity-linked insulin resistance without reduction in fat mass or leptin levels. Double FABP4-FABP5 null mice have lower fat mass and leptin levels. Conversely, transgenic mice that overexpress KLBP in fat cells under the aP2 promoter, have elevated fat cell mass and are more insulin resistant than are wild type animals. We hypothesize that FABPs stimulate lipolysis via interaction with HSL and that the disruption of the interaction results in a quantitative difference in both the molecular species and abundance of fatty acids that are lipolyzed from the fat cell. Such changes in lipolysis-derived fatty acids affect the availability of fatty acids linked to the development of insulin resistance. To test this hypothesis we propose to: Specific Aim A: Evaluate lipolysis in vivo and in situ from low and high fat fed mice harboring either disrupted FABP genes or the aP2-KLBP transgene. Correlate the rate and extent of lipolysis to insulin resistance in such animal models. Specific Aim B: Examine the physical interaction of FABPs with HSL in vitro. Assess the role of FABPs in regulating the activity of HSL and map the interaction domains of the two proteins. Specific Aim C: Assess the association of FABPs with HSL in 293 cells and 3T3-L1 adipocytes using fluorescence resonance energy transfer. Examine the phosphorylation/translocation of HSL and its relationship to FABP association.